The remarkable adsorption increase of water-preadsorbed microporous carbons of different micropore widths w under a water vapor atmosphere upon introduction of supercritical methane or argon was measured at 303 K. The adsorption increment reached a maximum 1-2 h after the introduction of supercritical gas, decreasing gradually to the equilibrium after about 80 h. The precise quantitative analysis of gas-phase components after and before the introduction of supercritical gas using gas chromatography showed that this uptake was ascribed to additional adsorption of water induced by the introduction of supercritical gas. A variety of experimental data lead to the following hysteresis-associated adsorption mechanism of the water vapor. The transient compression of water vapor upon the introduction of supercritical gas gives rise to an increase of the partial pressure of water near the microporous carbon, inducing a predominant adsorption along the adsorption isotherm of water. Upon relaxation of the transient compression, additionally adsorbed water is desorbed along the desorption scanning curve.